JButton actionPerformed 方法给出 Null

Question

所以我正在编写一个程序，允许用户为我设计的 "Novel" 和 "Magazine" class 的属性输入文本值，但是我我的 actionListener 有问题，每当我触发操作时它都会给我一个空指针异常。我的猜测是这与我有 switch 语句有关

switch(theList.getSelectedIndex()){
                case -1: JOptionPane.showMessageDialog(null, "You did not select an object type to create!", "Error!", JOptionPane.ERROR_MESSAGE);
                        break;
                case 0: createObject(theMagazine, titleField, authorField, publisherField, publisherLocationField, copyrightYearField, priceEnterField, ISBNEnterField);
                        JOptionPane.showMessageDialog(null, theMagazine.toString(), "This is The Magazine You Created!", JOptionPane.INFORMATION_MESSAGE);
                        break;
                case 1: createObject(theNovel, titleField, authorField, publisherField, publisherLocationField, copyrightYearField, priceEnterField, ISBNEnterField);
                        theNovel.setHardCover(Boolean.parseBoolean(JOptionPane.showInputDialog(null,"Is this Book a hard cover or not?","Question!",JOptionPane.QUESTION_MESSAGE)));
                        JOptionPane.showMessageDialog(null, theNovel.toString(), "This is The Magazine You Created!", JOptionPane.INFORMATION_MESSAGE);
                        break;
            }

基本上我想要它做的是能够为适当的属性值调用 createObject() 方法，然后显示 JOptionPane 的消息对话框，以显示使用这两个方法包含的 toString() 方法创建的对象 classes。我知道问题不是来自那些 classes 中的任何一个的方法语法，所以我不一定了解问题出在哪里。

这是源代码：

import javax.swing.*;
import javax.swing.border.*;
import java.awt.event.*;
import java.awt.*;
@SuppressWarnings({"serial"})
public class GUI extends JFrame{

static Magazine theMagazine;
static Novel theNovel;

public static void main(String[] args){
    final String[] objectTypes = {"Magazine","Novel"};
    final Border textFieldBorder = BorderFactory.createLineBorder(Color.black, 1);
    final Font theFont = new Font("Times New Roman", 0, 25);
    final JLabel typeSelect = new JLabel("Please select the literature type!");
    final JLabel titleEnter = new JLabel("Enter the title here!");
    final JLabel authorEnter = new JLabel("Enter the Author here!");
    final JLabel publisherEnter = new JLabel("Enter the publisher here!");
    final JLabel pubLocationEnter = new JLabel("Enter the Publishers location here");
    final JLabel enterCopyrightYear = new JLabel("Enter the copyright year here!");
    final JLabel priceEnter = new JLabel("Enter the price here!");
    final JLabel ISBNEnter = new JLabel("Enter the ISBN here!");    

    JList<String> theList = new JList<String>(objectTypes);
    theList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
    theList.setLayoutOrientation(JList.HORIZONTAL_WRAP);
    theList.setVisibleRowCount(-1);
    theList.setBorder(textFieldBorder);
    theList.setFont(theFont);

    JTextField titleField = new JTextField();
    titleField.setBorder(textFieldBorder);
    titleField.setFont(theFont);
    titleField.setHorizontalAlignment(JTextField.CENTER);
    JTextField authorField = new JTextField();
    authorField.setBorder(textFieldBorder);
    authorField.setFont(theFont);
    authorField.setHorizontalAlignment(JTextField.CENTER);
    JTextField publisherField = new JTextField();
    publisherField.setBorder(textFieldBorder);
    publisherField.setFont(theFont);
    publisherField.setHorizontalAlignment(JTextField.CENTER);
    JTextField publisherLocationField = new JTextField();
    publisherLocationField.setBorder(textFieldBorder);
    publisherLocationField.setFont(theFont);
    publisherLocationField.setHorizontalAlignment(JTextField.CENTER);
    JTextField copyrightYearField = new JTextField();
    copyrightYearField.setBorder(textFieldBorder);
    copyrightYearField.setFont(theFont);
    copyrightYearField.setHorizontalAlignment(JTextField.CENTER);
    JTextField priceEnterField = new JTextField();
    priceEnterField.setBorder(textFieldBorder);
    priceEnterField.setFont(theFont);
    priceEnterField.setHorizontalAlignment(JTextField.CENTER);
    JTextField ISBNEnterField = new JTextField();
    ISBNEnterField.setBorder(textFieldBorder);
    ISBNEnterField.setFont(theFont);
    ISBNEnterField.setHorizontalAlignment(JTextField.CENTER);

    JButton createObject = new JButton("Create an Object!");
    createObject.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent event){
            switch(theList.getSelectedIndex()){
                case -1: JOptionPane.showMessageDialog(null, "You did not select an object type to create!", "Error!", JOptionPane.ERROR_MESSAGE);
                        break;
                case 0: createObject(theMagazine, titleField, authorField, publisherField, publisherLocationField, copyrightYearField, priceEnterField, ISBNEnterField);
                        JOptionPane.showMessageDialog(null, theMagazine.toString(), "This is The Magazine You Created!", JOptionPane.INFORMATION_MESSAGE);
                        break;
                case 1: createObject(theNovel, titleField, authorField, publisherField, publisherLocationField, copyrightYearField, priceEnterField, ISBNEnterField);
                        theNovel.setHardCover(Boolean.parseBoolean(JOptionPane.showInputDialog(null,"Is this Book a hard cover or not?","Question!",JOptionPane.QUESTION_MESSAGE)));
                        JOptionPane.showMessageDialog(null, theNovel.toString(), "This is The Magazine You Created!", JOptionPane.INFORMATION_MESSAGE);
                        break;
            }
        }
    });
    JButton closeProgram = new JButton("Close the program!");
    closeProgram.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent event){
            System.exit(0);
        }
    });

    JPanel thePanel = new JPanel();
    thePanel.setLayout(new GridLayout(9,2));

    JFrame theFrame;

    //first row
    thePanel.add(typeSelect);
    thePanel.add(theList);

    //second row
    thePanel.add(titleEnter);
    thePanel.add(titleField);

    //third row
    thePanel.add(authorEnter);
    thePanel.add(authorField);

    //fourth row
    thePanel.add(publisherEnter);
    thePanel.add(publisherField);

    //fifth row
    thePanel.add(pubLocationEnter);
    thePanel.add(publisherLocationField);

    //sixth row
    thePanel.add(enterCopyrightYear);
    thePanel.add(copyrightYearField);

    //seventh row
    thePanel.add(priceEnter);
    thePanel.add(priceEnterField);

    //eighth row
    thePanel.add(ISBNEnter);
    thePanel.add(ISBNEnterField);

    //ninth row
    thePanel.add(createObject);
    thePanel.add(closeProgram);

    theFrame = new JFrame();
    theFrame.add(thePanel);
    theFrame.setTitle("Literature Creator!");
    theFrame.setSize(500,500);
    theFrame.setVisible(true);
    theFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    theFrame.setResizable(false);   

}
public static void createObject(Literature theLiterature, JTextField titleField, JTextField authorField, JTextField publisherField, JTextField publisherLocationField, 
                        JTextField copyrightYearField, JTextField priceEnterField, JTextField ISBNEnterField){
    theLiterature.setTitle(parseField(titleField));
    theLiterature.setAuthor(parseField(authorField));
    theLiterature.setPublisher(parseField(publisherField));
    theLiterature.setPublisherLocation(parseField(publisherLocationField));
    theLiterature.setCopyrightYear(parseNumField(copyrightYearField));
    theLiterature.setPrice(parseNumField(priceEnterField));
    theLiterature.setISBN(parseNumField(ISBNEnterField));
}
public static String parseField(JTextField field){
    if(field.getText() == null){
        JOptionPane.showMessageDialog(null, "Error! " + field.getName() + " contains no text!", "Error", JOptionPane.ERROR_MESSAGE);
        System.exit(0);
    }
    return field.getText();
}
public static int parseNumField(JTextField field){
    if(field.getText() == null){
        JOptionPane.showMessageDialog(null, "Error! " + field.getName() + " contains no text!", "Error", JOptionPane.ERROR_MESSAGE);
        System.exit(0);
    }
    try{
        Integer.parseInt(field.getText());
    }catch(NumberFormatException nfe){
        JOptionPane.showMessageDialog(null, "Error! Value in field " + field.getName() + " was not a valid number!", "Error!", JOptionPane.ERROR_MESSAGE);
        System.exit(0);
    }
    return Integer.parseInt(field.getText());
}

}

以下是错误消息的内容：http://pastebin.com/HRi9jbSD

非常感谢任何帮助！

Answer 1

默认情况下 Java 将 null 分配给新定义的变量。这个静态变量

static Magazine theMagazine;

当您将

传递给 createObject(...) 方法时，

为 null。

您需要创建新的杂志对象（或以某种方式加载它）并将其分配给 theMagazine 变量。

JButton actionPerformed 方法给出 Null

JButton actionPerformed method giving Null

java

events

swing

event-handling

actionlistener